According to the Centers for Disease Control, traffic accidents cost the United States over $99 Billion annually with more than 2 million people being injured in more than 5.5 million crashes. This includes the nearly 40,000 people who die from crash-related injuries. The additional monetary cost of the related damage to the vehicles and property involved is unknown.
Vehicular collisions include:                1) the front-impact collision, which involves two vehicles colliding head-on or a vehicle crashing directly into a barrier such as a telephone pole;        2) the offset crash in which only part of the front of a vehicle impacts with another vehicle or a barrier;        3) the side-impact crash wherein a vehicle crashes into the side of another vehicle;        4) the rear-end collision wherein a vehicle crashes into the back of another vehicle;        5) collision with a pedestrian.        
A pre-crash system has been defined in Wikipedia as “an automobile (or other vehicle) system that is designed to reduce the severity of an accident. Most are known as forward collision warning systems that use radar and sometimes radar sensors to detect an imminent crash. Depending on the system used, they may warn the driver, pre charge the brakes, inflate the seat belts for extra support, move the passenger seat to a safer position, and fold up the rear head rest for whip lash, retract the seat belts removing excess slack and automatically apply partial or full braking to minimize the crash severity.” Sensors can also be microwave, ultrasonic, laser and optical depending on the system used. The terms pre-crash system, collision avoidance system, pre-collision avoidance system, pre-collision system and forward collision warning system are often used interchangeably.
Because it is the kinetic energy of the forward moving vehicle that causes the damage in a collision, pre-crash systems rely on the vehicle's brakes to slow down or stop the vehicle in time to avoid the collision or at least to minimize its impact.
Not all pre-crash systems are alike. In Volkswagen's system, once the severity of collision is determined the vehicle's airbags are deployed and the system automatically unlocks all doors, disconnects the battery terminal from the alternator cable, shuts off the fuel supply and turns on the warning hazards and interior lights.
Audi's system works in four phases. Initially, it provides a warning of an impending accident, activates the hazard warning lights, closes the side windows and the sunroof, and tightens the front seat belts. The warning is followed by light braking which is strong enough to get the driver's attention. Then, autonomous braking is initiated at a rate of 3 m/s. Thereafter, the vehicle is decelerated at 5 m/s followed by deceleration at full braking power, roughly half a second before impact. To lessen the adverse consequences of a rear end collision, the windows and the sunroof are closed, the seat belts are prepared for impact and the optional memory seats are moved forward to protect the occupants of the vehicle.
Ford's system provides a warning through a heads-up display and if the driver does not react, the system precharges the brakes and increases the brake assist sensitivity to maximize the driver's braking performance.
In Honda's system a radar based system monitors the situation ahead and provides automatic braking if the driver does not react to both a warning displayed in the instrument panel and a tightening of the seat belts. Honda has also developed a night vision system, which highlights pedestrians in front of the vehicle by alerting the driver with an audible chime and a visual warning.
Nissan offers a laser-based system, which pre-pressures the braking system so maximum force can be applied early.
Toyota offers a radar based system which preemptively tightens the seat belts to remove any slack and pre-charges the brakes using brake assist to give the driver maximum stopping power when the driver depresses the brake pedal when the system determines that a frontal collision is unavoidable.
The Lexus division of Toyota offers a rearward facing millimeter-wave radar mounted in the rear bumper which adjusts the active head restraints by moving them upward and forward to avoid the risk of whip lash injuries if an imminent rear collision is detected.
The known pre-crash systems are focused on accident avoidance through the use of the vehicle's brakes while protecting the vehicle's occupants. However, these systems do not address what happens when the vehicle is unable to stop in time and a collision is about to occur.
Many vehicles have been designed to include a crumple zone. The crumple zone is that part of the vehicle intentionally designed to deform during a collision in order to absorb the kinetic energy of the vehicle. Naturally, a crumple zone by design sacrifices a significant portion of the vehicle to protect the occupants of the vehicle.
To summarize, the pre-crash systems that are currently known can vary in complexity and function from the relatively straight-forward vision-based crash avoidance technology, which helps prevent collisions by applying the brakes in the event of an impending front end collision, to a technically complex radar-based system which monitors the traffic situation ahead and initially provides automatic partial braking and then provides a maximum braking force seconds before an impact occurs. These pre-crash systems all involve applying the brakes to stop the vehicle, if possible, to prevent a collision with an object. If a pre-crash system is not able to stop the vehicle in time to prevent a collision, the crumple zones of one or both vehicles in a collision are designed to absorb the kinetic energy of the colliding vehicle and thereby protect the occupants from being injured. Thus, when a collision is unavoidable, both vehicles will likely suffer severe damage in the process of absorbing the kinetic energy of the forward moving vehicle.